348 research outputs found
Characterization of the second- and third-order nonlinear optical susceptibilities of monolayer MoS using multiphoton microscopy
We report second- and third-harmonic generation in monolayer MoS
as a tool for imaging and accurately characterizing the material's nonlinear
optical properties under 1560 nm excitation. Using a surface nonlinear optics
treatment, we derive expressions relating experimental measurements to second-
and third-order nonlinear sheet susceptibility magnitudes, obtaining values of
m V and for the first time for
monolayer MoS, m V.
These sheet susceptibilities correspond to effective bulk nonlinear
susceptibility values of m V and
m V, accounting for the sheet
thickness. Experimental comparisons between MoS and graphene are
also performed, demonstrating 3.4 times stronger third-order sheet
nonlinearity in monolayer MoS, highlighting the material's
potential for nonlinear photonics in the telecommunications C band.Comment: Accepted by 2D Materials, 28th Oct 201
Second harmonic microscopy of monolayer MoS2
We show that the lack of inversion symmetry in monolayer MoS2 allows strong
optical second harmonic generation. Second harmonic of an 810-nm pulse is
generated in a mechanically exfoliated monolayer, with a nonlinear
susceptibility on the order of 1E-7 m/V. The susceptibility reduces by a factor
of seven in trilayers, and by about two orders of magnitude in even layers. A
proof-of-principle second harmonic microscopy measurement is performed on
samples grown by chemical vapor deposition, which illustrates potential
applications of this effect in fast and non-invasive detection of crystalline
orientation, thickness uniformity, layer stacking, and single-crystal domain
size of atomically thin films of MoS2 and similar materials.Comment: 6 pages, 4 figure
Characterization of the second- and third-harmonic optical susceptibilities of atomically thin tungsten diselenide
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-018-28374-1We report the first detailed characterization of the sheet third-harmonic optical susceptibility, χ(3)s, of tungsten diselenide (WSe2). With a home-built multiphoton microscope setup developed to study harmonics generation, we map the second and third-harmonic intensities as a function of position in the sample, pump power and polarization angle, for single- and few-layers flakes of WSe2. We register a value of |χ(3)s| ≈ 0.9 × 10-28 m3 V-2 at a fundamental excitation frequency of ℏω = 0.8 eV, which is comparable in magnitude to the third-harmonic susceptibility of other group-VI transition metal dichalcogenides. The simultaneously recorded sheet second-harmonic susceptibility is found to be |χ(2)s| ≈ 0.7 × 10-19 m2 V-1 in very good agreement on the order of magnitude with recent reports for WSe2, which asserts the robustness of our values for |χ(3)s|.Y.W.H. acknowledges scholarship support from NGS. G.E. acknowledges financial support from National
Research Foundation of Singapore (NRF Research Fellowship NRF-NRFF2011-02 and medium-sized centre
programme) and Ministry of Education of Singapore (AcRF Tier 2 MOE2015-T2-2-123). V. M. P. acknowledges
fnancial support from Ministry of Education of Singapore (FRC AcRF Tier 1 R-144-000-386-114). J.C.V.G.
acknowledges fnancial support from CA2DM through National Research Foundation of Singapore (NRF-CRP
Grant No. R-144-000-295-281)
Ultra-strong nonlinear optical processes and trigonal warping in MoS2 layers
Nonlinear optical processes, such as harmonic generation, are of great interest for various applications, e.g., microscopy, therapy, and frequency conversion. However, high-order harmonic conversion is typically much less efficient than low-order, due to the weak intrinsic response of the higher-order nonlinear processes. Here we report ultra-strong optical nonlinearities in monolayer MoS2 (1L-MoS2): the third harmonic is 30 times stronger than the second, and the fourth is comparable to the second. The third harmonic generation efficiency for 1L-MoS2 is approximately three times higher than that for graphene, which was reported to have a large χ (3). We explain this by calculating the nonlinear response functions of 1L-MoS2 with a continuum-model Hamiltonian and quantum mechanical diagrammatic perturbation theory, highlighting the role of trigonal warping. A similar effect is expected in all other transition-metal dichalcogenides. Our results pave the way for efficient harmonic generation based on layered materials for applications such as microscopy and imaging.We acknowledge funding from the Academy of Finland (Nos: 276376, 284548, 295777, 298297, and 304666), TEKES (NP-Nano, OPEC), Royal Academy of Engineering (RAEng) Research Fellowships, Fondazione Istituto Italiano di Tecnologia, the Graphene Flagship, ERC grants Hetero2D, Nokia Foundation, EPSRC Grants EP/K01711X/1, EP/K017144/1, EP/L016087/1, AFOSR COMAS MURI (FA9550-10-1-0558), ONR NECom MURI, CIAN NSF ERC under Grant EEC-0812072, and TRIF Photonics funding from the state of Arizona and the Micronova, Nanofabrication Centre of Aalto University
How strong is the Second Harmonic Generation in single-layer monochalcogenides? A response from first-principles real-time simulations
Second Harmonic Generation (SHG) of single-layer monochalcogenides, such as
GaSe and InSe, has been recently reported [2D Mater. 5 (2018) 025019; J. Am.
Chem. Soc. 2015, 137, 79947997] to be extremely strong with respect to bulk and
multilayer forms. To clarify the origin of this strong SHG signal, we perform
first-principles real-time simulations of linear and non-linear optical
properties of these two-dimensional semiconducting materials. The simulations,
based on ab-initio many-body theory, accurately treat the electron-hole
correlation and capture excitonic effects that are deemed important to
correctly predict the optical properties of such systems. We find indeed that,
as observed for other 2D systems, the SHG intensity is redistributed at
excitonic resonances. The obtained theoretical SHG intensity is an order of
magnitude smaller than that reported at the experimental level. This result is
in substantial agreement with previously published simulations which neglected
the electron-hole correlation, demonstrating that many-body interactions are
not at the origin of the strong SHG measured. We then show that the
experimental data can be reconciled with the theoretical prediction when a
single layer model, rather than a bulk one, is used to extract the SHG
coefficient from the experimental data.Comment: 8 pages, 4 figure
Tungsten disulfide-gold nanohole hybrid metasurfaces for nonlinear metalens in the visible region
Recently, nonlinear hybrid metasurface comes into an attractive new concept
in the research of nanophotonics and nanotechnology. It is composed of
semiconductors with an intrinsically large nonlinear susceptibility and
traditional plasmonic metasurfaces, offering opportunities for efficiently
generating and manipulating nonlinear optical responses. A high second-harmonic
generation (SHG) conversion efficiency has been demonstrated in the
mid-infrared region by using multi-quantum-well (MQW) based plasmonic
metasurfaces. However, it has yet to be demonstrated in the visible region.
Here we present a new type of nonlinear hybrid metasurfaces for the visible
region, which consists of a single layer of tungsten disulfide (WS2) and a
phased gold nanohole array. The results indicate that a large SHG
susceptibility of ~0.1 nm/V at 810 nm is achieved, which is 2~3 orders of
magnitude larger than that of typical plasmonic metasurfaces. Nonlinear
metalenses with the focal lengths of 30 {\mu}m, 50 {\mu}m and 100 {\mu}m are
demonstrated experimentally, providing a direct evidence for both generating
and manipulating SH signals based on the nonlinear hybrid metasurfaces. It
shows great potential applications in designing of integrated, ultra-thin,
compacted and efficient nonlinear optical devices, such as frequency
converters, nonlinear holography and generation of nonlinear optical vortex
beam
Infrared Energy Conversion in Plasmonic Fields at Two-Dimensional Semiconductors
Conversion of infrared energy within plasmonic fields at two-dimensional, semiconductive transition metal dichalcogenides (TMD) through plasmonic hot electron transport and nonlinear frequency mixing has important implications in next-generation optoelectronics. Drude-Lorentz theory and approximate discrete dipole (DDA) solutions to Maxwell’s equations guided metal nanoantenna design towards strong infrared localized surface plasmon resonance (LSPR). Excitation and damping dynamics of LSPR in heterostructures of noble metal nanoantennas and molybdenum- or tungsten-disulfide (MoS2; WS2) monolayers were examined by parallel synthesis of (i) DDA electrodynamic simulations and (ii) near-field electron energy loss (EELS) and far-field optical transmission UV-vis spectroscopic measurements. Susceptibility to second-order nonlinear frequency conversion processes, X(2), for monolayer MoS2 and WS2 were measured to be 660±130 pm V-1 and 280±18 pm V-1, respectively, by Hyper Rayleigh Scattering. Femtosecond conversion of resonant irradiation to injection of plasmonic hot electrons into the TMD were measured in EELS at a maximum of 11±5% quantum efficiency for an optimized physicochemical Au-WS2 junction. Measured nonlinear second harmonic generation (SHG) from a ca. 1 μm MoS2 monolayer was enhanced 17-84% by local electric field augmentation from a single 150 nm Au nanoshell to a conversion efficiency of up to 0.023% W-1. Capacitive coupling between nanoshells arranged into dimers further augmented SHG activity from MoS2. New theoretical and experimental insights into energy conversion interactions between coupled plasmonic and excitonic materials spanning the linear and nonlinear optical regimes were established towards their implementation as an optoelectronic platform
Direct Observationof DegenerateTwo-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films
The optical nonlinearity of WS2, MoS2 monolayer and few-layer films was
investigated using the Z-scan technique with femtosecond pulses from the
visible to the near infrared. The dependence of nonlinear absorption of the WS2
and MoS2 films on layer number and excitation wavelength was studied
systematically. WS2 with 1~3 layers exhibits a giant two-photon absorption
(TPA) coefficient. Saturation of TPA for WS2 with 1~3 layers and MoS2 with
25~27 layers was observed. The giant nonlinearity of WS2 and MoS2 is attributed
to two dimensional confinement, a giant exciton effect and the band edge
resonance of TPA
- …